Single mode optical fiber having multi-step core structure and method of fabricating the same

A single mode optical fiber comprising:

• a central core having a predetermined first radius from a central axis and a first refractive index;
• a first outer core adjacent the central core, the first outer core having a second radius larger than the first radius and a second refractive index smaller than the first refractive index;
• a second outer core adjacent the first outer core, the second outer core having a third radius larger than the second radius and a third refractive index smaller than the second refractive index;
• and
• a cladding adjacent the second outer core, the cladding having a fourth radius larger than the third radius and a fourth refractive index smaller than the third refractive index, wherein the refractive index distributions of the central core, the first outer core, and the second outer core are one of refractive index distributions expressed by the following equation:
• n ⁡ ( r ) = n 1 ⁡ ( 1 - Δ ⁢ ⁢ ( r a ) α )
• wherein r is the radius, a is the radius of a core, α is the refractive index profile parameter, n1 is the refractive index at the center of the optical fiber, Δ=(n1−n2)/n1, and n2 is the cladding index, characterized in that:
• when α is equal to 1, the refractive index profile is triangular;
• when α is equal to 2, the refractive index profile has a hill shape;
• and
• when α is equal to ∞, the refractive index profile is stepped, and
• further characterized in that the ratios of refractive index difference of the central core, the first outer core, and the second outer core with respect to the refractive index of the cladding are 0.74-0.81%, 0.23-0.30%, and 0.13-0.17%, respectively.